1. Field of the Invention
The present invention relates to an analog keyboard of a video display appliance, particularly to an analog keyboard competent to perform a key input with encoding switches provided in a video display appliance.
2. Description of the Prior Art
Analog keyboards are conventionally positioned on the front lower part of a monitor of a video display appliance, as shown in FIG. 1A. Push-button switches K1 to K8 are used for manipulation of an analog keyboard. Each push-button switch has a screen control function. Thus, the user can control the display state of the picture on the screen by selecting different push-button switches, i.e., an adjustment of the size of the picture, control of the pin cushion raster appearing on the screen, balancing the picture in horizontal and vertical directions. For instance, the user can adjust the left and right balances of the picture by selecting the push-button switch K1 for the adjustment of balancing, and then the push button K7 or K8 for increase or decrease adjustment.
FIG. 1B is a block diagram showing the construction of the conventional analog keyboard. Referring to FIG. 1B, the conventional analog keyboard comprises a plurality of push-button switches K1 to K8 connected in parallel; a plurality of resistors R0 to R8 for dividing the power supply voltage Vcc into voltages corresponding to each push-button switch K1 to K8 connected therebetween; and an analog-to-digital converter 1 for converting the divided power supply voltage Vcc to an analog signal. The analog-to-digital converter 1 may be provided in the microcomputer of the video display appliance.
If the user manipulates a push-button switch, the divided power supply voltage Vcc is applied as an analog signal to the input terminal ADCin of the analog-to-digital converter 1. If the push-button switch K2 is switched on, the input voltage Vi applied to the input terminal ADCin of the analog-to-digital converter 1 can be defined by the following equation. ##EQU1##
The analog-to-digital converter 1 converts the voltage Vi to a digital signal. The microcomputer in the video display appliance recognizes this digital value and thus detects the switched state of the push-button switch K2, thereby outputting a control signal for controlling the display state of the picture.
However, each push-button switch employed in the conventional analog keyboard has only a single binary control function corresponding to the on/off state of the switch. Employing a great number of push-button switches to control the display state of the picture is disadvantageous in monitor design, and is liable to cause confusion in selecting appropriate switches.